Temporohyoid osteoarthropathy (THO), also known as otitis media-interna, is a progressive disease of the middle ear and temporohyoid joint that affects the stylohyoid and petrous temporal bones. THO affects all ages, breeds and gender of horses, worldwide.
THO is thought to be caused by an inner or a middle ear infection of hematogenous orgin that spreads to the associated bones, causing them to thicken and the temporohyoid joint to fuse. Other possibilities for its onset are from guttural pouch infection, nonseptic osteoarthritis, or from extension of otitis media/externa. Once the temporohyoid joint fuses and the associated bones thicken, the resulting forces produced during swallowing, teeth floating, oral or dental examinations, combined head and neck movements, or during vocalizing can potentially result in fractures of the petrous part of the temporal bone, causing dysfunction of the facial nerve (CN VII) and vestibulocochlear nerve (CN VIII).
Initial, early clinical signs of THO in horses include frequent ear rubbing, refusal to take the bit, head tossing, resistance to digital pressure around the base of the ears or on the basihyoid bone, refusing the position the head properly when saddled, and other nonspecific behavioral changes. Most horses also develop signs of facial nerve damage, such as inability to close the eyes, paresis or paralysis of the ear on the affected side, decreased tear production, and deviation of the upper lip away from the affected side. Horses that are unable to close their eyes and experience decreased tear production often develop corneal ulcers, keratitis, and keratoconjunctivitis.